Economically disadvantaged, black, and obese women have poorer breast cancer survival rates than other women, and Hispanic women may also have unfavorable breast cancer survival rates compared to non- Hispanic whites have also been demonstrated. There is evidence that the quality of breast cancer chemotherapy and radiation therapy affects disease-specific recurrence and mortality rates and that these quality measures vary by socioeconomic status (SES), race, ethnicity, and obesity status. However, no population-based studies have shown the impact of these quality measures on breast cancer survival rates, and no studies have estimated the effect of disparities in quality on disparities in outcomes. The proposed research attempts to address these gaps in our knowledge and understanding of disparities in breast cancer survival in vulnerable populations. The study hypotheses are that the quality of breast cancer treatment varies according to non-clinical factors and that observed disparities in recurrence and survival are reduced when controlling for the quality of breast cancer care. Detailed clinical and treatment data and long-term outcomes will be collected in a population-based sample of 3,000 women in the Los Angeles County and Detroit Metropolitan area Surveillance, Epidemiology, and End Results Registries. The sample will be stratified by site, race/ethnicity, stage, treatment modalities received, and vital status. Analyses will investigate the links between patient characteristics and quality of care and between quality of care and patient outcomes. More specifically, the analyses will assess the effect of quality of care on outcomes, quantify differences in treatment according to race, ethnicity, SES, and obesity status, and identify the dimensions of quality that generate disparities in outcomes. Quality of care will focus on those aspects of treatment that influence breast cancer outcomes appropriate use and dose intensity of chemotherapy, appropriate use of adjuvant hormonal therapy, and appropriate use, dose, and field of radiation therapy. Our findings will enable the development of interventions to eliminate disparities in breast cancer outcomes that target the most critical disparities in breast cancer treatment. Identifying the dimensions of quality that generate disparities in outcomes will help us understand better what practices represent best care and will provide an opportunity to address the disparities in outcomes.